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Dehydrated Culture Media

Sector: Clinical

GBS AGAR BASE (ISLAM)

Code: CM0755

For the isolation and detection of Group B streptococci (GBS) in clinical specimens.

Typical Formula*

gm/litre

Proteose peptone

23.0

Soluble starch

5.0

Sodium dihydrogen phosphate

1.5

Di-sodium hydrogen phosphate

5.75

Agar

10.0

pH 7.5 ± 0.1 @ 25°C

 
* Adjusted as required to meet performance standards 

Directions
Suspend 45.2g in 1 litre of distilled water and bring to the boil to disolve completely. Sterilise by autoclaving at 121°C for 15 minutes. Cool to 50°C and aseptically add 50ml of sterile inactivated Horse Serum‡. Mix well and pour into Petri dishes.

Sterile Inactivated Horse Serum
Hold sterile Horse Serum (Oxoid SR0035) at 56°C for 30 minutes.

Description
GBS Agar Base is based on the formulation described by Islam1. The medium is designed to exploit the ability of most group B streptococci (GBS) to produce orange/red pigmented colonies when incubated under anaerobic conditions.

Group B streptococci are a recognised cause of serious neonatal infection acquired from the infected mother. A review 2 of national data over an 8 year period by the Public Health Laboratory Service showed that group B streptococci accounted for 29.5% of all reports of neonatal bacterial meningitis with organisms being isolated from CSF and blood.

Group B streptococci may also be isolated from adults infected in a variety of sites.

The pigment of group B streptococci has characteristics of a carotenoid 3 and was first noted by Lancefield in 1934 in nine of twenty-four strains grown anaerobically. Modifications of media1,4,5 have improved the proportion of pigmented strains to about 97%. Noble et al.6 reported that in their studies 99.5% of beta-haemolytic GBS strains produced pigment. GBS Agar also supports growth of other genital bacteria that cause perinatal infections1, e.g. anaerobic streptococci, Bacteroides and Clostridium species.

Colonies of group B streptococci are 0.5-1mm in diameter, round, entire and pigmented orange/red after 24-48 hours anaerobic incubation. Other organisms able to grow on this medium do not produce the orange/red pigment.

de la Rosa et al.7 demonstrated the pigment-enhancing effect of trimethoprim/sulphonamides added to their medium. Work carried out in the Oxoid laboratories has shown that this pigment-enhancing effect can also be demonstrated around a sulphonamide disc placed on the inoculated plate. Standard discs of SF300 or SF500 can be used for this purpose. No inhibition of growth occurs and the enhanced pigment effect is clearly seen over a radius of 10-20mm.

Technique
1. Swabs should be collected into Stuart’s Transport Medium CM0111 and processed within 1/2-2 hours of collection8.
2. Inoculate the swab on to the surface of GBS Agar.
3. If desired, apply a disc containing 300 or 500 of Sulphafurazole on to an area of the plate where growth can be expected to be moderately profuse. These discs are available from Oxoid.
4. Incubate the plates anaerobically at 35°C for 24-48 hours.
5. Report all orange/red pigmented colonies as presumptive group B streptococci.
6. Identity can be confirmed using an Oxoid Streptococcal Grouping Kit DR0585 or Oxoid Dryspot Streptococcal Grouping Kit DR0400

Storage conditions and Shelf life
Store the dehydrated medium at 10-30°C and use before the expiry date on the label.
Store the prepared plates at 2-8°C away from light.

Appearance
Dehydrated medium: Straw coloured, free-flowing powder
Prepared medium: Straw coloured gel

Quality control

Positive controls:Expected results:
Streptococcus agalactiae NCTC 9993 Good growth orange pigmented colonies
Streptococcus agalactiae ATCC® 13813*Good growth non-pigmented colonies
Negative control: 
Enterococcus faecalis ATCC® 29212*Good growth non-pigmented colonies
* This organism is available as a Culti-loop®

Precautions
The medium must be at its correct pH value to ensure good pigmentation.
Some strains of Group B streptococci do not produce pigmented colonies.
Do not hold the molten medium any longer than necessary.

References
1. Islam A. K. M. S. (1977) Lancet i 256-257 (letter).
2. PHLS Communicable Disease Report (1954) CDR 84/38, 3-6.
3. Merrit K. and Jacobs N. J. (1978) J. Clin. Microbiol. 8. 105-107.
4. Fallon R. J. (1974) J. Clin. Pathol. 27. 902-905.
5. Merrit K. and Jacobs N. J. (1978) J. Clin. Microbiol. 4. 379-380.
6. Noble A. M., Bent J. M. and West A. B. (1983) J. Clin. Pathol. 36. 350-352.
7. de la Rosa M., Villareal R., Vega D., Miranda C. and Martinezbrocal A. (1983) J. Clin. Microbiol. 18. 779-785.
8. Islam A. K. M. S. (1981) J. Clin. Pathol. 34. 78-81.

 
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